310 M. F. Dreyer on the 



initial parts of differentiated skeletons with adult primitive 

 forms. With the shells of Thalamophora this has been 

 carried out in several special cases ; and in the case of the 

 Radiolaria, from their much greater differentiation it is pos- 

 sible to a much greater extent and with more profit. In these 

 cases comparative anatomy and ontogeny coincide, an advan- 

 tage in morphological investigation which cannot be too 

 highly appreciated, but which, unfortunately, like the compa- 

 rative treatment of the Rhizopoda in general, has hitherto by 

 no means received sufficient attention. 



Having now become acquainted with some of the most 

 important points in the structure of the shells of the E-hizo- 

 poda, it remains for us to give an explanation of these 

 phenomena. Here, of course, we can only have to do with a 

 preliminary attempt to throw some light upon the setiology of 

 the enormous form-labyrinth of the E-hizopoda, for even an 

 approximately complete solution of this difficult problem still 

 lies in the far distance. 



The chief cause of the form-types of the soft body and of 

 the shell is to be sought in the mode of life of the Rhizopoda 

 under consideration. Rhizopoda with shells belonging to the 

 perforate form-type and with pseudopodia radiating uniformly 

 on all sides w^ill live free and rotating in the water. The 

 monaxonic and amphitect shells of the pylomatic form-type 

 will belong to Rhizopoda w^iich, in swimming or creeping, 

 maintain a definite, perpendicular principal axis. The eudi- 

 pleural development, lastly, owes its origin to creeping in a 

 particular direction, just in the same way as in the example 

 of the Polyclada already adduced in this connexion. 



The morphological evolution or the specific character of 

 the form-types recurs, as has already been mentioned, in 

 exactly analogous development throughout, independently of 

 conditions of relationship and shell-material. With regard 

 to the perforate form -type, on account of its undifferentiated 

 character, there is not much to be said in this respect ; and 

 here we have chiefly to consider the above-mentioned asso- 

 ciated phenomena of the formation of the pylom, such as oral 

 marginal ornaments of the pylom, apical spinosity, &c. The 

 specific evolution of the form-type once selected is, as has 

 been said, independent of the shell-material ; in the selection 

 of the form-type itself, however, the latter plays an important 

 part, and this applies in a still higher degree to the growth- 

 Vype, inasmuch as the structural material plays a positively 

 determinant part with respect to the mode of growth of the 

 Rliizopod shell. 



The most important materials here coming under con- 



